Method of reducing the organic carbon content of production waste water in the course of the preparation of concentrated solutions of anionic organic compounds

ABSTRACT

The present invention relates to a method of reducing the total organic carbon (TOC) content of waste water in the course of the preparation of concentrated solutions or suspensions of anionic organic compounds, which method comprises increasing the concentration of an aqueous solution or suspension of an anionic organic compound in the form of its free acid or its alkali metal salt, having a salt content of less than 5% of extraneous salt by weight based on the total solution or suspension, by microfiltration, ultrafiltration and/or nanofiltration, a) the membrane pore size being so selected that compounds having molecular weights in the range from 300 to 1000 Daltons or higher are retained, and b) the content of anionic compound in the concentrate being so adjusted to from 10 to 50% by weight that the total organic carbon (TOC) content of the permeate is less than 0.5% by weight, based on the total permeate, and c) optionally, after increasing the concentration of a suspension, dissolving the anionic organic compound by adding a suitable base. The invention relates also to the solutions and suspensions prepared by that means and to the use of such solutions or suspensions in dyeing or optically whitening paper or textile materials.

The present invention relates to a method of reducing the total organiccarbon (TOC) content of production waste water in the course of thepreparation of concentrated solutions of anionic organic compounds, tothe solutions or suspensions prepared by that means and to their use,anionic organic compounds being understood to be, especially,fluorescent whitening agents and dyes and also intermediates for thepreparation thereof.

The production of chemical substances such as, for example, dyes andfluorescent whitening agents gives rise to waste waters that are notbiologically degradable or that can be biologically degraded only withdifficulty. In the case of the starting materials, in contrast, it isfrequently still possible to achieve good biological elimination. Thereis accordingly a two-fold interest in minimising the amount of producttaken up into the waste water stream: on the one hand, to keep theproduct yield as high as possible and, as a result, to increase theeconomic viability of the process and, on the other hand, to avoidlaborious treatment of the waste water whilst nevertheless achieving awaste water quality that meets statutory outflow conditions, especiallywith regard to refractory constituents. If the untreated waste water isunable to meet those conditions, laborious purification steps arefrequently necessary, for example waste water incineration, wetoxidation, anodic electro-oxidation or chemical oxidation using H₂O₂ orozone. Frequently, additional concentration operations and preliminaryde-salting steps are also necessary.

Most of the refractory content of waste water from production of anionicorganic compounds usually consists of the end products of the chemicalreaction themselves, which is why the main priority must be to ensurethat those substances do not pass into the waste water stream.

The standard method for isolating the end products of anionic organiccompounds is usually to add large amounts of salt to the reactionsolution (salting-out), as a result of which the anionic organiccompound is precipitated and can be separated out, for example, using afilter press. The filter cake is rinsed, for example, with demineralisedwater in order to wash out impurities and excess salt. In the process,further amounts of the desired product pass into solution andaccordingly enter the waste water stream, or the fine particles passthrough the filter material.

The salt load and the total carbon content of the waste water areconsiderable in that method and have to be removed again by laboriousmeans as mentioned above, which can scarcely be done economically. Infact, elevated salt concentrations have an adverse effect on biologicalpurification, resulting in an even poorer level of degradation of theorganic carbon.

The filter cake, on the other hand, is dried and, using suitable aids,is converted into the desired commercial form, which may be a powder ora solution, especially an aqueous solution.

In recent years, the use of concentrated aqueous solutions, for exampleof dyes and fluorescent whitening agents, has increased in importance,more specifically because of the advantages of such solutions over thecorresponding powder forms. The use of solutions avoids the difficultiesassociated with dust formation and frees the users from thetime-consuming and often difficult task of dissolving the powder inwater. The use of concentrated solutions has also been encouraged by thedevelopment of continuous processes for dyeing or optically whiteningpaper because it is advantageous in such processes to introduce thesolution directly into the hollander or the processing device inquestion. Liquid components can be dispensed in measured amounts inproduction much more simply than solids.

EP-A-992 547 proposes a method of increasing the concentration of dyeand whitening agent suspensions by means of microfiltration at a poresize of from 0.05 to 40 μm, wherein de-salting is carried out at thesame time. The suspension usually contains 1-25% salt by weight. Animprovement in the TOC value of the waste waters is not described.

International Application WO 0190257 A2 likewise describes a method ofincreasing the concentration of dye and whitening agent suspensions bymeans of ultrafiltration, which method uses smaller pore sizes. In thatcase too, the starting material consists of waste waters having a saltcontent, which are de-salted and concentrated by means of the process.An influence on the TOC value of the waste waters is, likewise, notdescribed therein.

It has now been found, surprisingly, that it Is possible to dispensewith salting-out of the anionic organic compounds from the reactionmixture if that mixture is subjected to microfiltration, ultrafiltrationor nanofiltration. As a result, the salt loads associated withsalting-out are absent from the waste water, and the TOC value,especially the refractory carbon content, amounts to only a fraction ofthat in the case of conventional working-up, as a result of which thewaste waters can be directly passed to the biological purificationplant. There is no need for problematic waste waters to be collected andsubjected to collective working-up, with the result that each productionlocation generally can introduce the resulting waste water directly intothe biological purification plant (“end of pipe” solution). In fact, inmany cases the product quality is found to be improved.

The present invention relates to a method of reducing the total organiccarbon (TOC) content of waste water in the course of the preparation ofconcentrated solutions or suspensions of anionic organic compounds,which method comprises

-   increasing the concentration of an aqueous solution or suspension of    an anionic organic compound in the form of its free acid or its    alkali metal salt, having a salt content of less than 5% of    extraneous salt by weight based on the total solution or suspension,    by microfiltration, ultrafiltration and/or nanofiltration,-   a) the membrane pore size being so selected that compounds having    molecular weights in the range from 300 to 1000 Daltons or higher    are retained, and-   b) the content of anionic compound in the concentrate being so    adjusted to from 10 to 50% by weight that the total organic carbon    (TOC) content of the permeate is less than 0.5% by weight, based on    the total permeate, and-   c) optionally, after increasing the concentration of a suspension,    dissolving the anionic organic compound by adding a suitable base.

A free acid is understood to be a compound containing an SO₃H and/orCOOH group; alkali metal salts thereof are the salts with Li, Na or K,especially with Li or Na.

As already mentioned hereinbefore, the solutions or suspensions are freefrom salts from a salting-out process. Any small amounts of salt presentmay be present only from the actual synthesis steps.

As anionic organic compound, preference is given to the use of afluorescent whitening agent, a dye or an intermediate for thepreparation thereof. Special preference is given to fluorescentwhitening agents.

Preference is given to a method wherein a sulfo- and/orcarboxy-group-containing fluorescent whitening agent from one of thefollowing classes is used: bis-triazinylamino-stilbenes,bis-triazolyl-stilbenes, bis-styryl-biphenyls orbis-benzofuranylbiphenyls, bis-benzoxalyl derivatives,bis-benzimidazolyl derivatives, coumarin derivatives or pyrazolinederivatives.

The fluorescent whitening agent contains especially at least 2 sulfogroups.

Special preference is given to fluorescent whitening agents of formula1, 2, 3, 4 or 5

wherein

-   R₁ and R₂ are each independently of the other —OH, —Cl, —NH₂,    O—C₁-C₄alkyl, —O-aryl, —NH—C₁-C₄alkyl, —N(C₁-C₄alkyl)₂,    —N(C₁-C₄alkyl)(C₁-C₄hydroxyalkyl), —N(C₁-C₄hydroxyalkyl)₂, —NH-aryl,    morpholino or S—C₁-C₄alkyl(aryl), and aryl is phenyl or naphthyl,    each of which may be substituted by 1 or 2 sulfo groups;

wherein

-   R₃ and R₄ are each independently of the other hydrogen, C₁-C₄alkyl,    phenyl or a radical of formula

wherein

-   R₅ is hydrogen, Cl or SO₃M; and-   R₆ is —CN, —SO₃M, —S(C₁-C₄alkyl)₂ or S(aryl)₂;

wherein

-   R₇ is hydrogen, —SO₃M, —O—C₁-C₄alkyl, —CN, —Cl, —COO—C₁-C₄alkyl or    CON(C₁-C₄alkyl)₂;

wherein

-   R₈ is hydrogen, —C₁-C₄alkyl, —Cl or —SO₃M; and-   R₉ and R₁₀ are each independently of the other hydrogen, C₁-C₄alkyl,    —SO₃M, —Cl or —O—C₁-C₄alkyl;-   M is hydrogen, an alkali metal or an ammonium cation, and-   n is a number 1, 2 or 3.

Specifically preferred fluorescent whitening agents are indicated below.

The fluorescent whitening agents are known and, for the most part,commercially available.

Another preferred group of anionic organic compounds comprises dyes thathave at least one sulfonic acid group and/or carboxylic acid group andare selected from the following classes of dyes: metal-free ormetal-containing mono-, dis- and poly-azo dyes, pyrazolone,thioxanthone, oxazine, stilbene, formazan, anthraquinone, nitro,methine, triphenylmethane, xanthone, naphthazarin, styryl, azastyryl,naphthoperinone, quinophthalone and phthalo-cyanine dyes.

Within that group, preference is given to azo dyes having at least onesulfo group, especially to the so-called azo direct dyes listed in theColour Index, Third Edition, Volume 2 (The Society of Dyers andColourists, 1971).

In addition to those azo dyes, preference is given also tophthalocyanine dyes that have at least one sulfo group.

An especially preferred group of azo dyes comprises those of formula

wherein KK is the radical of a coupling component.

Preference is given to coupling components of formula (H)

wherein

-   Y₁ and Y₂ are each independently of the other ═O, ═NH or    ═N—C₁-C₄alkyl,-   Y₃ is ═O, ═S, ═NR or ═N—CN, R being hydrogen or C₁-C₄alkyl, and-   R₁ and R₂ are each independently of the other hydrogen,    unsubstituted or substituted alkyl or unsubstituted or substituted    phenyl.

Especially preferred specific dyes are Direct Yellow 11, Direct Yellow 6and Direct Orange 15.

The suitable dyes are likewise known and commercially available.

When the anionic organic compounds are intermediates, they arepreferably aromatic sulfonic acids that still carry one or more furthersubstituents from the series amino, nitro, alkyl and hydroxy.

Special preference is given to 2-amino-5-hydroxynaphthalene-7-sulfonicacid, 4-amino-toluene-2-sulfonic acid, dehydroparathiotoluidinesulfonicacid, 4,4′-diaminostilbene-2,2′-disulfonic acid,4,4′-dinitrostilbene-2,2′-disulfonic acid,4,4′-diamino-diphenylamine-2-sulfonic acid and 4-nitrotoluene-2-sulfonicacid.

An important advantage of the method according to the invention is that,before treatment, no additional salt is added to the synthesis solutionor suspension for the purpose of precipitating the product. Therefore,only very small amounts of salt are present (<5%), which do not have tobe separated out by means of further separation steps. The salts thatare still present are mainly alkali metal halides, alkali metal sulfatesor alkali metal hydrogen sulfates, especially NaCl, KCl, Na₂SO₄ andNaHSO₄.

Preference is given to starting from an aqueous synthesis solution orsuspension having a salt content of less than 1.0% by weight, especiallyless than 0.5% by weight, based on the solution or suspension, which maystill comprise starting materials and subsidiary products or otherimpurities in addition to the anionic organic compound.

Process technology distinguishes between the following pressure-drivenmembrane separation methods: filtration, microfiltration,ultrafiltration, nanofiltration and reverse osmosis, although theseparation methods partly overlap in respect of the size of particles tobe separated and process conditions such as the pressure to be applied.

Typically, the size of particles retained is from 0.1 to 10 μm or largerin microfiltration, from 0.01 to 0.1 μm and larger in ultrafiltrationand from 0.001 to 0.01 μm or larger in nanofiltration. The retention issubstantially governed by the pore size of the membranes used, thepressure conditions and the flow conditions. Accordingly, the pressurerange is typically from 1 to 3 bar in microfiltration, from 2 to 10 barin ultrafiltration and from 7 to 40 bar in nanofiltration. Suchseparation methods and their implementation in process technology aredescribed in detail, for example, in the Hochschulkurs Membranprozesse(University Course in Membrane Processes), Part 1 and Part 2, of theInstitut für Verfahrenstechnik of the RWRH Aachen, edited by Th. Melin,2000.

Because, in the case of ultra- and nano-filtration, the pore sizedistribution can be determined only by laborious means and, in addition,the particle dimensions of the substances separated are generally notknown, the molecular weight of the retained component is selected as thecharacteristic variable. Because ascertaining such a separationcharacteristic in its entirety is laborious, it is usually found to besufficient, when characterising the membrane, to indicate the limit atwhich 90% or 95% of the molecules having a particular molecular weightare retained (molecular weight cut off, MWCO).

In accordance with the invention, the nanofiltration membranes andmethods are so selected that the MWCO is, depending on the product, from300 to 1000 Daltons, preferably from 300 to 800 Daltons and especiallyfrom 300 to 500 Daltons. For ultrafiltration, membranes, preferablyceramic membranes, are so selected that the pore size is in the rangefrom 10 to 50 nm, especially 20 nm.

In the context of the present invention, all three membrane methods maybe used, it being possible for different or identical modules to beconnected in series.

Especially preferred methods are nanofiltration and ultrafiltration,especially nanofiltration.

True solutions are always present in the case of nanofiltration, whereassuspensions are present in the case of ultrafiltration andmicrofiltration.

The membranes used in ultra- and micro-filtration are exclusively poremembranes. Whether or not a particle is retained by the membrane,besides being dependent upon the operating conditions, is especiallydependent upon the size and structure of the particle relative to thesize and structure of the membrane pores; essentially, convective masstransfer takes place. A different limiting case is present in reverseosmosis, in which only diffusive transfer of the components passing overtakes place. In that respect, nanofiltration is located betweenultrafiltration and reverse osmosis.

Membranes suitable for all the methods are commercially available. Theymay comprise membranes made from organic polymers such as, for example,polystyrene, polyester, polyacrylonitrile, polypropylene, celluloseacetate or polyvinylidene fluoride. Those active membranes are usuallymounted on a large-pore support layer.

Inorganic membranes are made from, for example, zirconium oxide,aluminium oxide or titanium oxide or mixtures thereof.

Preferably, there are used, for microfiltration and ultrafiltration, aceramic membrane or an acid-resistant organic membrane and, fornanofiltration, an organic membrane.

The ultrafiltration and microfiltration method is preferably carried outin the acid range at a pH of <4.5 or in the highly alkaline range at apH of >9.

The nanofiltration method is preferably carried out in the pH range from5 to 11.

Microfiltration, ultrafiltration or nanofiltration is carried outpreferably at from room temperature to about 95° C., especially from 30to 85° C.

Nanofiltration is carried out especially at from 30 to 55° C.

Microfiltration, ultrafiltration or nanofiltration is carried outpreferably at a pressure of from 2 to 40 bar.

Preferred pressure ranges for micro- and ultra-filtration are from 2 to10 bar, especially from 2 to 5 bar. Nanofiltration is carried out atpreferably from 7 to 40 bar, especially from 12 to 30 bar.

Optionally, when the product whose concentration is increased by micro-or ultra-filtration is present in the form of a suspension it may beconverted into a liquid solution, which is carried out by addingsuitable bases.

Bases suitable therefor are, for example, LiOH, NH₄OH or an organicamine.

Suitable organic amines are, for example, a C₄-C₁₂trialkylamine,C₄-C₁₂diamine, C₂-C₁₅-alkanolamine or polyglycolamine.

Preference is given to the use of LiOH, NH₄OH or an alkanolamine.

The dye or whitening agent solutions obtained may be used directly inthat form or, where appropriate, after dilution. However, they may alsobe dried in conventional manner and used in the form of powder orgranules.

The invention accordingly relates also to the solutions or suspensionsof anionic organic compounds obtained in accordance with the methoddescribed hereinbefore and to the use thereof in dyeing or opticallywhitening paper or textiles or in the synthesis of anionic organiccompounds.

The following Examples illustrate the Invention.

EXAMPLE 1

The compound of formula (E) is prepared in the manner described in U.S.Pat. No. 4,717,502 or in DD 55 668.

2 kg of the reaction mixture from the final reaction step (comprisingabout 13.5% of dissolved product by weight and also small amounts ofNaCl (<5%) from the various synthesis steps), without first being saltedout and after being diluted with 20% of deionised water by weight, aredirectly, in a nanofiltration, washed with 1.6 kg of deionised water andthen concentrated to about 1.55 kg. A composite membrane from NittoDenko, Type NTR-7450, is used. The total membrane surface area is about190 cm², with a pressure of 23 bar being used. The cut-off is in therange from 400 to 500 g/mol. The permeate has a TOC value of 0.1%, ofwhich 80% can be biologically degraded satisfactorily. It can be passeddirectly to the biological purification plant.

Comparison Test 1

2 kg of the reaction mixture which was sent directly for the increase inconcentration mentioned in Example 1 are salted out using 0.25 kg ofNaCl. The thick suspension is transferred to a suction filter and iswashed with 0.42 litres of demineralised water. The waste water has aTOC value of 0.9%, of which 10% are biologically degradable and 90% arerefractory. The waste water must be subjected to special treatmentbefore it can be passed to the biological purification plant.

1. A method of reducing the total organic carbon (TOC) content of wastewater in the course of the preparation of concentrated solutions orsuspensions of anionic organic compounds, which method comprisesincreasing the concentration of an aqueous solution or suspension of ananionic organic compound in the form of its free acid or its alkalimetal salt, having a salt content of less than 5% of extraneous salt byweight based on the total solution or suspension, by nanofiltration, a)the membrane pore size being so selected that compounds having molecularweights in the range from 300 to 1000 Daltons or higher are retained,and b) the content of anionic compound in the concentrate being soadjusted to from 10 to 50% by weight so that the total organic carbon(TOC) content of the permeate is less than 0.5% by weight, based on thetotal permeate, and c) optionally, after increasing the concentration ofa suspension, dissolving the anionic organic compound by adding asuitable base.
 2. A method according to claim 1, wherein the anionicorganic compound used is a fluorescent whitening agent, a dye or anintermediate for the preparation thereof.
 3. A method according to claim2, wherein a sulfo— and/or carboxy-group-containing fluorescentwhitening agent from one of the following classes is used:bis-triazinylamino-stilbenes, bis-triazolyl-stilbenes,bis-styryl-biphenyls or bis-benzofuranylbiphenyls, bis-benzoxalylderivatives, bis-benzimidazolyl derivatives, coumarin derivatives orpyrazoline derivatives.
 4. A method according to claim 3, wherein thefluorescent whitening agent contains at least 2 sulfo groups.
 5. Amethod according to claim 2, wherein a fluorescent whitening agent offormula 1, 2, 3, 4 or 5 is used

wherein R₁ and R₂ are each independently of the other —OH, —Cl, —NH₂,—O—C₁-C₄ alkyl, —O—aryl, —NH—C₁-C₄alkyl, —N(C₁-C₄alkyl)₂,—N(C₁-C_(4 alkyl)(C) ₁-C₄hydroxyalkyl), —N(C₁-C₄hydroxyalkyl) ₂,—NH—aryl, morpholino or S—C₁-C₄alkyl(aryl), and aryl is phenyl ornaphthyl, each of which may be substituted by 1 or 2 sulfo groups;

wherein R₃ and R₄ are each independently of the other hydrogen, C₁-C₄alkyl, phenyl or a radical of formula

wherein R₅ is hydrogen, Cl or SO₃M; and R₆ is —CN, —SO₃M, —S(C₁-C₄alkyl)or —S(aryl);

wherein R₇ is hydrogen, —SO₃M, —O—C₁-C₄alkyl, —CN, —Cl, —COO—C₁-C₄alkylor CON (C₁-C₄alkyl)₂;

wherein R₈ is hydrogen, —C₁-C₄alkyl, —Cl or —SO₃M; and R₉ and R₁₀ areeach independently of the other hydrogen, C₁-C₄alkyl, —SO₃M, —Cl or—O—C₁-C₄alkyl; M is hydrogen, an alkali metal or an ammonium cation, andn is a number 1, 2 or
 3. 6. A method according to claim 5, wherein thereis used a fluorescent whitening agent


7. A method according to claim 2, wherein there is used a dye having atleast one sulfonic acid group and/or carboxylic acid group from thefollowing classes of dyes: metal-free or metal-containing mono-, dis-and poly-azo dyes, pyrazolone, thioxanthone, oxazine, stilbene,formazan, anthraquinone, nitro, methine, triphenylmethane, xanthone,naphthazarin, styryl, azastyryl, naphthoperinone, quinophthalone andphthalocyanine dyes.
 8. A method according to claim 7, wherein there isused an azo dye having at least one sulfo group.
 9. A method accordingto claim 8, wherein there is used a dye of formula

wherein KK is the radical of a coupling component.
 10. A methodaccording to claim 9, wherein there is used a dye of formula (G) whereinKK is a coupling component of formula

wherein Y₁ and Y₂ are each independently of the other ═O, ═NH or═N—C₁-C₄ alkyl, Y₃ is ═O, ═S, ═NR or ═N—CN, R being hydrogen orC₁-C₄alkyl, and R₁ and R₂ are each independently of the other hydrogen,unsubstituted or substituted alkyl or unsubstituted or substitutedphenyl.
 11. A method according to claim 7, wherein the dye Direct Yellow11, Direct Yellow 6 or Direct Orange 15 is used.
 12. A method accordingto claim 2, wherein there is used, as anionic intermediate, an aromaticsulfonic acid that still carries one or more further substituentsselected from the group consisting of amino, nitro, alkyl and hydroxy.13. A method according to claim 12, wherein there is used2-amino-5-hydroxynaphthalene-7-sulfonic acid, 4-aminotoluene-2-sulfonicacid, dehydroparathiotoluidinesulfonic acid,4,4′-diaminostilbene-2,2′-disulfonic acid,4,4′-dinitrostilbene-2,2′-disulfonic acid,4,4′-diamino-diphenylamine-2-sulfonic acid or 4-nitrotoluene-2-sulfonicacid.
 14. A method according to claim 1, which comprises starting froman aqueous synthesis solution or suspension having a salt content ofless than 1.0% by weight, based on the solution or suspension.
 15. Amethod according to claim 1, which comprises using an organic membranefor nanofiltration.
 16. A method according to claim 1, wherein thenanofiltration is carried out at from room temperature to about 95° C.17. A method according to claim 1, wherein the nanofiltration is carriedout at a pressure of from 2 to 40 bar.
 18. A method according to claim1, wherein after nanofiltration, LiOH, NH₄OH or an organic amine isadded to the suspension obtained.
 19. A method according to claim 18,wherein as organic amine there is used a C₄-C₁₂-trialkylamine,C₄-C₁₂diamine, C₂-C₁₅alkanolamine or polyglycolamine.